Sounds to me like you have a bad speedeo gear in the trannie try taking in out on the trans goto the dealer get a new one then install That should take care of it

Nissan_King_84

1984 Nissan King Cab 4x4
http://www.geocities.com/nowheres

 

Any idea how to figure out whether it's the speed sensor itself or the speedometer unit? Several mechanics have said it could be either.

 

pull the gear out of spedeo gear out of the transmission and if the teeth are mangled then there is the problem if not then the problem lies with the speedometer itself

Nissan_King_84

1984 Nissan King Cab 4x4
http://www.geocities.com/nowheres

 

Thanks. I cannot get the speedo sensor out of the tranny. After I remove the 10mm bolt, should the sensor and gear simply pull out of the hole or do I need to remove something else first like a clip or what appears to be the sensor housing on the side of the tranny? The housing appears to be really stuck on the side of the tranny.

 

I don't have my book with me on your truck but i'm assuming the spedeo cable Unscrews like mine does then if i remember yes you pull a 10 mm bolt then it should pull out as a complete assembly if not grab a pairt of needle nose plires and reach in the tranie and pull the gear out
it will take some slight prying there is oa O-Ring gasket on the unit plus probably years of grease mud around the unit

Nissan_King_84

1984 Nissan King Cab 4x4
http://www.geocities.com/nowheres